Method and system for planning/guiding alterations to a bone

ABSTRACT

A computer-assisted surgery system for guiding alterations to a bone, comprises a trackable member secured to the bone. The trackable member has a first inertial sensor unit producing orientation-based data. A positioning block is secured to the bone, and is adjustable once the positioning block is secured to the bone to be used to guide tools in altering the bone. The positioning block has a second inertial sensor unit producing orientation-based data. A processing system providing an orientation reference associating the bone to the trackable member comprises a signal interpreter for determining an orientation of the trackable member and of the positioning block. A parameter calculator calculates alteration parameters related to an actual orientation of the positioning block with respect to the bone.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a divisional of U.S. Pat. No. 8,718,820,filed on Aug. 17, 2012 which is a continuation of U.S. Pat. No.8,265,790, filed on Mar. 25, 2009 which claims priority on U.S.Provisional Patent Application No. 61/039,184, filed on Mar. 25, 2008,and U.S. Provisional Patent Application No. 61/100,173, filed on Sep.25, 2008.

FIELD OF THE APPLICATION

The present application relates to computer-assisted surgery systemsand, more particularly, to instrumentation used for tracking orpositioning surgical tools during computer-assisted surgery.

BACKGROUND OF THE ART

Tracking of surgical instruments or tools is an integral part ofcomputer-assisted surgery (hereinafter CAS). The tools are tracked forposition and/or orientation in such a way that information pertaining tobodily parts is obtained. The information is then used in variousinterventions (e.g., orthopedic surgery, neurological surgery) withrespect to the body, such as bone alterations, implant positioning,incisions and the like during surgery.

The tracking systems may use different technologies, such as mechanical,acoustical, magnetic, optical and RF tracking. Depending on thetechnology used, different types of trackable references are fixed,permanently or temporarily, to the item that needs to be tracked. Forinstance, during Total Knee Replacement (TKR) surgery, trackablereferences are fixed to the limbs and to the different surgicalinstruments, and these trackable references are tracked by the trackingsystem. The CAS system calculates position and orientation dataassociated with the tracking, and the information displayed by thecomputer is used by the surgeon to visualize the position of theinstrument(s) being manipulated with respect to the limbs, or innumerical values.

Two types of tracking systems are commonly used. The active trackingsystems provide a transmitter as trackable reference on the tool to betracked, which transmitter emits signals to be received by a processorof the CAS system, which will calculate the position and/or orientationof the tool as a function of the signals received. The transmitters ofthe active tracking systems are powered, for instance by being wired tothe CAS system or by being provided with an independent power source, soas to emit signals.

Passive tracking systems do not provide active transmitters on the toolsas trackable references. The CAS system associated with passive trackinghas an optical sensor apparatus provided to visually detect opticalelements on the tools. The optical elements are passive, whereby nopower source is associated therewith.

In order to obtain values for position and/or orientation, the opticalelements must be in the line of sight of the optical sensor apparatus.Accordingly, with passive tracking systems, surgery takes place in agiven orientation as a function of the required visibility between theoptical sensor apparatus and the optical elements.

The trackable references currently used, whether active or passive, havea noticeable size depending on the technology used. For anelectromagnetic system, a casing is wired to the CAS system and issecured to the instrument or to the patient. For an optical system, atrackable reference generally comprises at least three optical elementsin order to provide six degrees of freedom (DOF). For instance, theoptical elements are light sources wired to the CAS system and forming ascalene triangle. The light sources can be individually fixed orassembled on a base. In this second construction, the assembly is largeand obstructive.

As an alternative, passive reflector spheres or patches can be usedinstead of light sources, and a light source is used to illuminate them(in the infrared spectrum).

Some factors must be considered when selecting a type of trackingsystem: the presence of wires in sterile zones for active trackablereferences; a line of sight required for navigation when using opticaltracking; the size of the trackable references in order to deliver therequired precision during surgery; the necessity for the surgeon tovisualize a computer screen for intraoperative alignment information;the necessity for the surgeon to digitize landmarks on bones in order tobuild coordinate systems; the difficulty in integrating current opticalor radio-frequency sensors in disposable instruments (such as cuttingguides) because of their volume. Electromagnetic tracking devices aresubject to distortions introduced by conventional orthopaedicinstruments which may be difficult to detect and may cause a loss inaccuracy. These tracking devices are used as general data input devices,digitizing points on patients or surgical instruments in order tocompute planes, point-to-point distances, planar angles, planardistances, etc., required during CAS.

No alternate miniaturized technologies with fewer than 6 DOF iscurrently used in orthopaedic CAS, while still providing the crucialinformation required to install orthopaedic implants. Such technologycould be directly integrated to instruments, thus reducing the need foran external tracking system, thereby resulting in enhanced ease-of-use.

SUMMARY OF THE APPLICATION

It is therefore an aim of the present application to provide a methodand system for planning/guiding alterations to bones which addressissues associated with the prior art.

Therefore, in accordance with the present disclosure, there is provideda computer-assisted surgery system for planning/guiding alterations to abone in surgery, comprising: a trackable member adapted to be secured tothe bone, the trackable member having a first inertial sensor unitproducing orientation-based data for at least two degrees of freedom inorientation of the trackable member; a positioning block adapted to besecured to the bone, with at least an orientation of the positioningblock being adjustable once the positioning block is secured to the boneto reach a selected orientation at which the positioning block is usedto guide tools in altering the bone, the positioning block having asecond inertial sensor unit producing orientation-based data for atleast two degrees of freedom in orientation of the positioning block; aprocessing system providing an orientation reference between the boneand the trackable member and comprising: a signal interpreter fordetermining an orientation of the trackable member and of thepositioning block from the orientation-based data; and a parametercalculator for calculating alteration parameters related to an actualorientation of the positioning block with respect to the bone as afunction of the orientation reference and of the orientation of thepositioning block.

Further in accordance with the present disclosure, there is provided amethod for planning/guiding alterations to a bone comprising: providinga trackable member secured to a bone, the trackable member having afirst inertial sensor producing orientation-based data for at least twodegrees of freedom in orientation for the trackable member; providing apositioning block secured to the bone, the positioning block having aninertial sensor unit producing orientation-based data for at least twodegrees of freedom in orientation for the positioning block, anorientation of the positioning block being adjustable with respect tothe bone; determining an orientation reference of the bone at least fromthe orientation-based data of the trackable member; and calculating bonealteration parameters from the orientation-based data of the positioningblock with respect to the orientation reference of the bone.

Still further in accordance with the present disclosure, there isprovided a caliper for determining a dimension of an object, comprising:a base having a known base length; arms pivotally mounted to ends of thebase, the arms each having a known arm length, and each having a freeend used to identify a limit point of the object to measure; an inertialsensor unit secured to at least the arms, the inertial sensor unitproducing orientation data pertaining to at least one degree of freedomin orientation of the arms in a plane in which the arms and the baselie; whereby the dimension between limit points is calculated from theknown base length and arm lengths and from the orientation data of thearms.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of a trackable CAS universalpositioning block according to an embodiment;

FIG. 2 is a front elevation view of the universal positioning block ofFIG. 1;

FIG. 3 is a side elevation view of a polyaxial mounting screw elementused to fasten the universal positioning block of FIG. 2 to a boneelement;

FIG. 4A is a side elevation view of the universal positioning block ofFIG. 1 mounted to a femur;

FIG. 4B is a side elevation view of the universal positioning block ofFIG. 1 mounted to a femur and the positioning body proximally displacedsuch that it abuts the femur;

FIG. 5 is a flow chart illustrating a method for planning/guidingalterations to a bone in computer-assisted surgery in accordance with anembodiment of the present disclosure;

FIG. 6 is a block diagram illustrating a computer-assisted surgerysystem for planning/guiding alterations to a bone in accordance withanother embodiment of the present disclosure;

FIG. 7 is a schematic view of a caliper in accordance with anotherembodiment of the present disclosure;

FIG. 8 is a perspective view of an axis-digitizing device as used in thecomputer-assisted surgery system of the present application, inaccordance with a first embodiment;

FIG. 9 is a perspective view of a positioning block in accordance withanother embodiment of the present application;

FIG. 10 is a perspective view of the positioning block of FIG. 9 asmounted to a bone;

FIG. 11 is a perspective view of an axis-digitizing device used with thecomputer-assisted surgery system of the present application, inaccordance with another embodiment;

FIG. 12 is a perspective view of a positioning block with trackingmember as secured to a tibia; and

FIG. 13 is perspective view of the positioning block with trackingmember of FIG. 12 from another standpoint;

FIG. 14 is a perspective view of a tracking member and spike trackingmember on the femur, in accordance with another embodiment of thepresent application;

FIG. 15 is a perspective view of spike tracking member supporting acutting guide;

FIG. 16 is a perspective view of the cutting guide as related to thetracking member; and

FIG. 17 is a perspective view of the cutting guide pinned to the femur.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 5, a method for planning/guiding alterations to a boneis generally illustrated at 1. The method 1 is used for instance tosubsequently alter bones in knee replacement surgery, in view ofinstalling knee joint implants on the femur and/or on the tibia.

Referring concurrently to FIGS. 5 and 6, the method 1 uses a positioningblock 10 (i.e., navigated cutting block), such as the positioning blocksdefined in United States Publication No. 2008/0065084, and United StatesPublication No. 2004/0039396, by the current assignee. The subjectmatter of both these references is incorporated herein by reference. Inboth these references, the positioning block is provided with an opticaltracker member that is visually tracked to serve as a guide forsubsequent alterations to the bone.

The present application features tracking members with inertia-basedtracking circuitry instead of the optical tracker member (i.e.,hereinafter inertial sensors). The tracking circuitry featuresmicro-electromechanical sensors (MEMS), gyroscopes, accelerometers orother types of sensors (electrolytic tilt sensors, compasses) to detectorientation changes, for instance in the positioning block, instead ofelectromagnetic (EM) transmitter/receiver coils or optically-detectablemembers. In one embodiment, the sensors are connected to an embeddedprocessor on the positioning block. The following sensors areconsidered, amongst other possibilities: tri-axial gyroscopic sensors inan orthogonal or semi-orthogonal configuration as well as tri-axialaccelerometer sensors in an orthogonal or semi-orthogonal configuration.The method for computing angles between the cutting block and the boneis different from conventional tracking systems: planar information andoptionally position information is obtained directly from the MEMSdevices rather than having to compute this information from the opticaltracking data. In other words, the inertial sensors provide at least twodegrees of freedom in orientation, and optionally up to three degrees offreedom in position.

By way of example, referring to FIG. 1, an embodiment of the universalpositioning block assembly 10 comprises generally a cutting tool guideelement or guide body member 12, a mounting member 14 and a MEMStracking circuit C. The main guide body 12 comprises a large centralaperture 18 for receiving the mounting member 14 therein. The guide body12 comprises cutting guide surfaces, such as the two drill guide holes36, which extend through the guide body 12. The guide body 12 alsoincludes means for engagement to a cutting guide, comprising, forexample, a pair of mounting points 38 having peg holes 40 that aredisposed on the top of the guide body, permitting engagement withanother drill/cutting guide block for example.

The mounting member 14 comprises a translation mechanism including afastener receiving mount element 24, which slides within the centralguide slot 22 disposed within the mounting member body 20. The fastenermount element 24 comprises a semi-spherically shaped bowl 26 which has athrough hole at the bottom thereof. The fastener mount element 24 isdisplaced relative to the mounting member body 20 by an endless screw28, engaged to the fastener mount element and extending through aninside-threaded hole 32 in the mounting member body 20. The translationscrew 28 is actuated by a screw head 30 such that rotation of the screwhead 30 causes the fastener mount element 24 to be translated within thecentral guide slot 22. The translation, or elevation, screw 28 therebyenables the entire positioning block to be raised or lowered, forinstance along an anterior-posterior axis when engaged to a distal endof a femur. The entire mounting member 14 additionally slides within thecentral aperture 18 of the guide body 12, generally permitting the guidebody to be displaced along a proximal-distal axis when the positioningblock is engaged to a distal end of a femur. A friction locking screw 34extends through the side of the guide body and engages the mountingmember 14, such that it can be retained in a selected position relativeto the guide body 12.

A polyaxial mounting screw 25, as best seen in FIG. 3, is used to mountthe universal positioning block 10 to the bone. The polyaxial screw 25comprises generally a main screw body 29 having threads on the outside,a shoulder portion 27, and a spherical screw head 31 having a pluralityof integrally formed individual petal elements 33. A central conicalscrew 35 is inserted through the center of the screw head, and whenengaged therein, forces the petal elements 33 outwards, thereby causingthem to press against the semi-spherical surface 26 of the fastenermount element 24. This consequently immobilizes the fastener mountelement 24 in position on the spherical polyaxial screw head 31, fixingit in position thereon. The petal elements 33 are slightly elasticallydeflectable and the polyaxial screw head 31 is sized such that the petalelements are forced slightly radially inward when the fastener mountingelement is pressed down overtop, and engaged to the screw head. Thisensure that once snapped in place, the fastener mount element 24, andsubsequently the entire positioning block assembly, can freely rotateabout the polyaxial screw head in three rotational degrees of freedom.Once the positioning block is aligned in the desired position, theconical screw 35 at the center of the polyaxial screw head 31 can betightened, thereby rotationally fixing the guide block assembly in placeon the polyaxial mounting screw 25. When the term polyaxial screw isused herein, it is to be understood that it comprises preferably a screwhaving a substantially spherical head. The spherical head permits a balland socket type joint to be created, when an element with a receivingsocket is engaged with the ball head of the polyaxial screw. Thespherical head preferably, but not necessarily, includes the individualpetal elements that are displaceable by the central conical screw inorder to provide a locking mechanism. Other mechanisms to lock themember with the receiving socket in a selected position on the head ofthe screw are equivalently possible.

As described hereinafter, the positioning block 10 with MEMS is used incombination with another MEMS tracker member 10′ that performs thedynamic tracking of the bone B. The MEMS tracker member 10′ is secureddirectly to the bone B (or soft tissue) to be in a fixed relation withthe bone B.

In another embodiment illustrated for instance in FIGS. 12 and 13, thepositioning block 10 with MEMS is used in an independent manner, wherethe mechanical axis measurements described hereinafter, or a portionthereof, are determined directly by the positioning block pinned on thebone, instead of through the use of the tracking member 10′ which may ormay not be present in this embodiment. Tracking circuitry (equivalent tothe tracking member 10′) is provided on both the fixed portion of thepositioning block (i.e., fixed to the bone), and the movable portion ofthe positioning block. Once the mechanical axis measurements aredetermined, the positioning block would then be used to perform theplanned bone cut(s), as further described below. Therefore, as thepositioning block 10 is secured to the bone, both the MEMS fixed to thebone and the MEMS of the movable portion of the positioning block 10 areinstalled.

Now that the MEMS positioning block 10 and the MEMS tracker member 10′are defined, the method 1 is described as used to plan alterations onthe femur at the knee, with reference being made to FIG. 5.

According to step 2 of the method, the MEMS tracker member 10′ issecured to the femur.

According to step 3 of the method, at least one axis of the femur isdigitized. For the femur, the axis is, for instance, the mechanical axispassing through a center of the femoral head and a central point betweenthe condyles at the knee. The axis can also be a rotational axis of thebone, pointing either in a medio-lateral or antero-posterior direction.

In order to digitize the mechanical axis, the femur is rotated about itsmechanical axis, and the movements are sensed by the MEMS trackingmember 10′ on the femur. By the sensing data collected by the MEMStracker member 10′ secured to the femur, a computer-assisted surgerysystem digitizes the mechanical axis of the femur and tracks themechanical axis through sensing data from the trackable member 10′.

Various methods are considered for the digitization of a mechanical axisfor the femur.

According to a first embodiment, an additional tracking member istemporarily secured to the femur at the entry point of the mechanicalaxis. By the weight of the patient, the pelvis of the patient is deemedto be in a fixed spatial position and orientation. The tracking memberat the entry point of the mechanical axis, also known as a spiketracking member, is of the type equipped with tracking circuitryproviding six-degree-of-freedom tracking data. With the tracking memberat the entry point, a given motion about the center of rotation of thefemur in the pelvis is performed (e.g., in a freehand manner). Themotion can be continuous, or decomposed in several displacements withstable positions in between them. The tracking data resulting from thegiven motion is used to calculate a position and orientation of thecenter of rotation of the femur. The mechanical axis is then defined aspassing through the center of rotation and the entry point (i.e., thespike tracking member). The orientation of the mechanical axis istransferred to the tracking member 10′. The spike tracking member maythen be removed, with MEMS tracking member 10′ kept on the femur for thesubsequent tracking of the mechanical axis of the femur.

Referring to FIG. 14, as an alternative to having a MEMS unit in thespike tracking member, a rigid link 50 may be provided between the spike51 and the tracking member 10′. In this case, the geometry of the rigidlink 50 is known such that the orientation of the spike 51 is calculableas a function of the tracking data from the tracking member 10′. Oncethe orientation of the mechanical axis of the femur is known andtransferred to the tracking member 10′, the rigid link 50 and spike 51may be removed form the femur.

Alternatively, the spike 51 may be used as an alternative to thepolyaxial screw to which the cutting guide 10 will be anchored. As theorientation and possibly the position of the spike 51/51′ are known, theorientation of the cutting guide 10 may be known as a function of thetracking of the tracking member 10′. Referring to FIG. 16, the spike 51′may be removed while the cutting guide 10 remains in place.

In a second embodiment, the spike tracking member has tracking circuitryproducing at least two-degree-of-freedom tracking data and linearaccelerations along three orthogonal axes. The spike tracking member 51′(FIG. 15) is positioned at the entry point of the mechanical axis on thefemur. In order to find the center of rotation, accelerative motions areperformed according to a freehand or constrained trajectory for thedistal part of the femur with respect to the immoveable pelvis. Thistrajectory can be spherical, linear or any other suitable pattern. Anorientation of the mechanical axis may then be computed from the trackedaccelerations and/or orientations of the femur. Once the orientation ofthe mechanical axis is known, the orientation of the mechanical axis istransferred to tracking member 10′ and the spike tracking member isremoved, and the tracking member 10′ is tracked so as to follow theorientation of the mechanical axis of the femur. As an alternative tohaving a MEMS unit in the spike tracking member, a rigid link may beprovided between the spike and the tracking member 10′, as illustratedin FIG. 14. In this case, the geometry of the rigid link is known suchthat the orientation of the spike is calculable as a function of thetracking data from the tracking member 10′. Once the orientation of themechanical axis of the femur is known and transferred to the trackingmember 10′, the rigid link and spike may be removed from the femur.

In yet another embodiment, a three-axis force sensor is positioned atthe entry point of the mechanical axis of the femur. A force is appliedto the three-axis force sensor, which force is measured by thethree-axis force sensor. The measurement of the force enablescalculation of the orientation of the mechanical axis of the femur. Theforce sensor may then be removed, whereby the tracking member 10′ tracksthe orientation of the mechanical axis.

In yet another embodiment, the orientation of the mechanical axis isdetermined using the tracking member 10′, and by fixing the femur at itsfemoral center of rotation and at the entry point of the mechanicalaxis. A rotation about these two fixed points is then performed, whichrotation is therefore about the mechanical axis of the femur. With thevariation in orientation of the tracking member 10′, the orientation ofthe mechanical axis is calculated with respect to the tracking member10′, from the tracking data.

Reference is made above to the entry point of the mechanical axis of thefemur. The entry point of the mechanical axis is known to be in thedepression above the inter-condylar notch region of the knee. As analternative, it is considered to use a template to align the entry pointwith the center of the medio-lateral axis of the femur at the knee.

Various methods are considered for the digitization of a rotational axisfor the femur.

According to a first embodiment, the rotational axis of the bone can bedetermined with the aid of an axis digitization device. The spiketracking member 51/51′ may be equipped with two flat surfaces that canbe simultaneously placed under both posterior condyles while the spiketracking member 51/51′ is being inserted at the entry point of themechanical axis. The axis-digitization device can be aligned eithervisually or mechanically with bone landmarks.

In a second embodiment, the knee joint is moved in a flexion andextension motion. Such motion can be continuous, or decomposed inseveral displacements with stable positions in between them. From thetracked orientation of the tracking members 10′ of the tibia and femur,the orientation of the rotation axis of the femur can be determined.

In yet another embodiment, the knee is be positioned in 90 degrees offlexion. From the orientation of the tracking members 10′ of the tibiaand the femur, along with the previously digitized mechanical axis ofthe tibia, the rotational axis of the femur can be computed.

In yet another embodiment, the leg is positioned in full extension sothat the rotational axes of the femur and tibia are aligned. From theorientation of the tracked members of both bones, and the previouslydigitized rotational axis of the tibia, the rotational axis of the femurcan be computed.

With the rotational axis and the mechanical axis, a plane incorporatingthe mechanical axis is known. This data is used as an orientationreference for the subsequent calculation of parameters.

According to step 4, the positioning block 10 is then secured to thefemur at the central point between the condyles, as set forth in UnitedStates Publication No. 2008/0065084, and United States Publication No.2004/0039396. The positioning block 10 may be installed on the femurprior to step 3. Other configurations of positioning blocks may be used,such as the ones shown in FIGS. 9 and 10 and in FIGS. 12 and 13 anddescribed in further detail hereinafter. It is considered to have thetracking member 10′ on the fixed portion of the positioning block 10.

It is pointed out that steps 2 and 3 of the method are part of step 4when the positioning block has MEMS on both its fixed portion andmovable portion, as described above. More specifically, the MEMS issecured to the bone (i.e., step 2) when the positioning block is securedto the bone, and both MEMS provide orientation data simultaneously.

According to step 5, the positioning block 10 is calibrated with respectto the mechanical axis. More specifically, the positioning block 10defines planes that will be used to guide the operator in resecting thebone, and these planes are aligned with respect to the mechanical axis.The orientation of the mechanical axis may be validated. A validationtool (not shown) may be used by being applied to the posterior condylesof the distal femur. A rotation about the posterior condyles is trackedrelative to the tracking member 10′, and used as rotational informationwhen distal cuts are performed on the femur.

FIGS. 4A and 4B show the universal positioning block assembly 10 mountedto the distal end of a femur 39 by the polyaxial screw 25. The degree ofmobility of the universal positioning block 10 permits significantsimplification of the surgical procedures employed in certain surgeries,such as total knee replacement surgery. As shown in FIG. 4A and in step4 of FIG. 5, the fastening of the positioning block 10 to the bone B ispreferably done using the polyaxial screw 25, which is first alignedwith the entrance point of the mechanical axis at the distal end of thefemur and introduced therein until its shoulder 27 touches the bone. Thefastener mount element 24 of the universal positioning block 10, as bestseen in FIG. 1 and FIG. 2, is snapped onto the head 31 of the polyaxialscrew. As mentioned previously, to reduce the invasiveness of theprocedure, the tracking member 10′ and the positioning block 10 may beinterconnected. The tracking member 10′ would be on the fixed portion ofthe positioning block 10′. According to this embodiment, no polyaxialscrew would be required.

It is considered to align the positioning block with the posteriorcondyles, using the validation tool described above. It is alsoconsidered to align the positioning block 10 such that the positioningblock 10 is aligned with the anterior-posterior axis of the femur. Morespecifically, the anterior-posterior axis of the femur is visuallyidentifiable at the knee by an anterior point and a posterior point,namely the trochlear groove (Whiteside's line) or, alternatively, theanterior-posterior axis may be aligned to the plane perpendicular toboth posterior condyles. Therefore, when the positioning block 10 issecured to the femur, with the anterior-posterior axes being aligned,the adjustments in orientation of the positioning block with respect tothe femur are limited to flexion-extension and varus-valgus, which maybe adjusted independently from one another. The positioning block 10 canalso be positioned with respect to a rotation relative to theanterior-posterior axis or the posterior condyles.

According to step 6, an orientation of the positioning block 10 ismanually adjusted, as a function of the alterations to be performed onthe femur. For instance, the various screws on the positioning block 10are used to adjust the orientation of the block, with varus/valgus andflexion/extension being adjusted independently from one another as aresult of a previous calibration of the orientation of the positioningblock 10 on the bone (step 5).

Step 6 of determining a desired position of the positioning block 10, ora portion thereof such as a reference surface 45 on the guide body 12,is done either by the CAS system itself, by the surgeon using the CASsystem as a guide or independently by the surgeon, in order to determinewhat final position the positioning block 10 should be moved into suchthat a drilled hole or a sawn cut can be made in the bone element at apredetermined location that is required for the installation of animplant. Step 6 comprises adjusting the orientation of the positioningblock 10 until it, or a portion thereof such as the reference surface 45of the guide body 12, is located in the desired orientation. This caninvolve rotatably adjusting the positioning block 10 relative to thebone element, using the tracking information to aid in the correctorientation in each axis of rotation. Three rotational degrees offreedom are thereby possible, and the entire positioning block 10 can beoriented in a desired plane, for example parallel to the distal cut tobe made in the femur. Step 4 can also include proximally displacing thepositioning block 10 in the direction 43 such that the proximal surface45 is translated from a position shown in FIG. 4A to a position shown inFIG. 4B, abutting the femur 39. As the head 31 of the polyaxial screw 25is distally spaced from the condyles 41 of the femur 39, the positioningblock 10 requires a reference point with respect to the bone such thatthe location of the distal cutting guide, which will be fixed to thepositioning guide block, will correctly correspond to the amount of bonewhich must be resected by the distal cut.

The proximal-distal translation of the guide block body 12 relative tothe mounting member 14 simplifies the referencing of the guide blockwith the femur. As the mounting member 14 is engaged in place on thehead of the polyaxial screw, it is fixed in a proximal-distal directionrelative to the bone. However, as the guide block body 12 can axiallyslide relative to the central mounting member 14 when the locking screw34 is disengaged, the tracked guide body portion 12 remains rotationallyfixed relative to the mounting member but can translate in theproximal-distal direction 43. This permits the guide body 12 to beproximally displaced until its proximal surface 45 directly abuts themost distal end of the condyles 41, as shown in FIG. 4B. By tighteningthe locking screw 34, the guide body 20 is retained in place on thecentral mounting member 14. The conical screw 33, as seen in FIG. 3,when tightened, fixes the positioning block 10 in place on the head 31of the polyaxial screw 25, thereby fixing the reference surface 45 inthe chosen desired position. The distal end of the femur, which isaccurately located by the tracked guide body 20 that is located by theCAS system, can then be used as a reference plane, from which theresection depth can be easily measured. The amount of bone resectedoften varies as a function of the type of implant line being used, andthe specific structure of the patient anatomy.

Further adjustment is also possible with the present universalpositioning block assembly 10. Step 6 of FIG. 5 also comprisestranslation of the entire positioning block assembly 10 relative to thepolyaxial screw 25, and therefore relative to the femur, in theanterior-posterior direction 47. By rotating the screw head 30, themounting member body 20, shown in FIG. 2, and consequently the entireguide block body 12 are displaced relative to the fastener mount element24 that is fixed to the polyaxial screw head 31. This affordssubstantially vertical adjustment of the positioning block if requiredby the specific procedure or the anatomy of the patient being operated.The positioning block can therefore be adjusted in five degrees offreedom, namely rotation about three rotational axes and translationalong two perpendicular axes, namely in directions 43 and 47 and inrotation if needed.

According to step 7, alteration parameters such as varus/valgus andflexion/extension and rotation are provided as calculated by the CAS asa function of the adjustments to the orientation of the positioningblock 10. The CAS receives the tracking of the mechanical axis from thetracker member 10′, as well as the orientation changes from the MEMStracking circuitry on the positioning block 10. Therefore, the CASdeducts motion of the femur from the orientation changes of thepositioning block 10 to calculate the implant parameters. The amount ofvarus/valgus and flexion/extension is updated in real-time on thepositioning block and displayed to the surgeon by a simple graphicalmeans. For example, an array of Light-Emitting Diodes (LEDs) can bepositioned on the positioning block or within the field of view of thesurgeon, such that a green light may be turned on when the angle isappropriate and stays red as long as the orientation is not appropriatein a particular plane.

Once a desired orientation is set, the positioning block 10 is used toguide the operator in resecting the femur as set forth in United StatesPublication No. 2008/0065084, and in United States Publication No.2004/0039396.

If no tracker member 10′ is used on the femur during the cuttingprocedure, it could still be installed after the cut has been made inorder to provide hip-knee-ankle angle (i.e., HKA) information later onduring the procedure. Once the cut has been made, a tracker member 10′would then be fixed to the femur and all coordinate system informationregistered to this tracker member 10′ for further measurements, such asHKA.

It is considered to use the positioning block to confirm the cut planesof the femur at the knee. More specifically, as the orientation of thepositioning block 10 is known in all three degrees of freedom, thepositioning block 10 may simply be brought into contact with the varioussurfaces of the knee so as to obtain an orientation of the cut planeswith respect to the tracking member 10′ and thus as a function of themechanical axis of the femur. This allows the measurement of anydeviations that may occur during the cutting process.

Referring to FIGS. 15 to 17, different configurations are illustratedfor the positioning block 10, tracking member 10′ and spike trackingmember 51′ with MEMS. In FIG. 15, there is illustrated the positioningblock 10 being connected to the tracking member 51′. In this case, thespike tracking member 51′ forms a rigid link with the positioning block10, whereby an orientation tracking of the positioning block 10 ispossible from the tracking data of the spike tracking member 51′.

Referring to FIG. 16, a linkage 53 is provided between the trackingmember 10′ and the positioning block 10. Therefore, once the orientationof the positioning block 10 is tracked with respect to the mechanicalaxis or other reference of the femur, the linkage 53 allows the fineradjustment of the orientation of the positioning block 10 with respectto the femur. The positioning block 10 features visual indicators, suchas flexion-extension and varus-valgus, in view of a plane being cut inthe bone using the positioning block 10. Referring to FIG. 17, oncesuitable parameters are attained (e.g., varus-valgus, flexion-extension,etc.), the positioning block 10 is anchored to the femur, for instanceusing the pins 52.

The method 1 is now described as used to plan alterations on the tibiaat the knee.

According to step 2, the MEMS trackable member 10′ is secured to thetibia (or soft tissue) so as to be in a fixed relation with respect tothe tibia. Another MEMS trackable member could be used, with a shapethat is more appropriate for use with the tibia.

Alternatively, the trackable member 10′ could be eliminated if dynamictracking is not used because the tibia or the femur is immobilized andall tracking is performed via the MEMS positioning block 10, asdescribed above.

According to step 3 of the method, an axis of the tibia is digitized.The axis is, for instance, the mechanical axis of the tibia. Accordingto a first embodiment, in order to digitize the mechanical axis, thetibia is moved about a reference point and the movements are sensed bythe MEMS tracking member 10′ on the tibia. From the sensing datacollected by the MEMS tracker member 10′ secured to the tibia, thecomputer-assisted surgery system digitizes the mechanical axis of thetibia and tracks the mechanical axis through sensing data from thetrackable member 10′. Whether it be for the femur or the tibia, the axesmay be digitized in a freehand manner by the operator, for instanceusing a fixed visual reference point, or relying on the operator's skillto minimize given movements of the bone during step 3.

In a second embodiment, referring to FIG. 8, an axis-digitizing device70 is illustrated, and may be used to determine the mechanical axis ofthe tibia. The axis-digitizing device 70 has a trough 71 and a MEMS unit72. The trough 71 is positioned on the anterior crest of the tibia, forinstance, directly on the soft tissue, which happens to be relativelythin on the anterior crest of the tibia. Also, the middle point of thetibial plateau (from medial to lateral) can be connected to the middlepoint of the ankle joint with self-centering devices. The middle pointof the tibial plateau can be connected to the 2^(nd) metatarsal bone viaa guide rod or a laser pointing device. It must be ensured that there isno relative movement between the device 70 and the tibia during step 3.This is readily accomplished since the registration process is performedrelatively quickly. The MEMS unit 72 is typically equipped withtwo-degree-of-freedom or three-degree-of-freedom tracking circuitry, orcalibrated to perform orientation tracking.

Various methods are considered for the digitization of a rotational axisfor the tibia.

According to a first embodiment, the rotational axis of the bone can bedetermined with the aid of an axis digitization device, such as the axisdigitizing device 70 (FIG. 8), or any other suitable device. Theaxis-digitization device can be aligned either visually or mechanicallywith bone landmarks.

In a second embodiment, the knee joint is moved in a flexion andextension motion. Such motion can be continuous, or decomposed inseveral displacements with stable positions in between them. From thetracked orientation of the tracking members 10′ of the tibia and femur,the orientation of the rotation axis of the tibia can be determined.

In yet another embodiment, the knee is be positioned in 90 degrees offlexion. From the orientation of the tracked members of the tibia andthe femur, along with the previously digitized mechanical axis of thefemur, the rotational axis of the tibia can be computed.

In yet another embodiment, the leg is positioned in full extension sothat the rotational axes of the femur and tibia are aligned. From theorientation of the tracked members of both bones, and the previouslydigitized rotational axis of the femur, the rotational axis of the tibiacan be computed. The rotational axis and the mechanical axis arecombined to form an orientation reference for the calculation ofalteration parameters.

According to step 4, the positioning block 10 is then secured to thetibia at a desired position, as set forth in United States PublicationNo. 2008/0065084, and United States Publication No. 2004/0039396. It ispointed out that the positioning block 10 may be installed on the tibiaprior to step 3.

An alternative embodiment of the positioning block is illustrated at 75in FIGS. 9 and 10. When the positioning block 75 is secured to thetibia, the anterior-posterior axis of the positioning block 75 isaligned with that of the tibia. More specifically, points that can beused to visually identify the anterior-posterior axis of the tibia arethe connection point of the posterior cruciate ligament, and the medialthird tubercle. Other anatomical landmarks that can be used to definethe tibia anterior-posterior axis are described hereinafter. The axisperpendicular to the line joining the most posterior points of the tibiaplateau is a first alternative to the tubercle-PCL axis. Secondly, akinematic analysis performed between the femur and the tibia, inflexion-extension, can give a unique flexion-extension axis where theperpendicular can be used as another alternative to the previouslydescribed AP axis. Similarly, the axis perpendicular to the femoralposterior condyle axis can be projected on the tibia, when the leg is infull extension, and used again as the third options. Another alternativeAP landmark would be the projection of the femoral mechanical axis onthe tibia, when the leg is in pure flexion i.e. 90 degrees.

With the positioning block 75 being secured to the tibia with theanterior-posterior axes of the tibia and the positioning block beingaligned, the positioning block 10/75 may only be moved in theflexion-extension orientation and in the varus-valgus orientation.

The positioning block 75 has a base 76 that is fixedly secured to thebone. A cutting guide 77 is pivotally mounted to the base 76 by a pivotjoint. The cutting guide 77 has a slot 78 into which a blade is insertedto perform cuts on the tibia. A MEMS unit 77 is integral with thecutting guide 77 so as to track the orientation of the cutting planes,and provides 3-DOF tracking to provide tracking data related to theorientation of the cutting guide 77. The positioning block 75 is securedto the bone by a first threaded rod 80. Once a desired varus-valgusorientation is reached using knob 80A (FIG. 10), rod 81 is used so as tosecure the base 76 to the bone in the varus-valgus orientation. Theflexion-extension orientation is then adjusted using knob 81A so as toreach a desired orientation of the cutting guide 77 in view of creatingthe cutting planes on the tibia. It is pointed out that the virtual cutplanes may be tracked as a function of the geometry of the slot 78 inthe positioning block 75. More specifically, the MEMS unit 75, or theprocessing system 101 may be provide with the data representing the cutplanes, such that secondary cut planes can be tracked to simulate thepositioning of an implant on the bone.

According to step 5, the positioning block 10 is calibrated with respectto the mechanical axis. More specifically, the positioning block 10defines planes that will be used to guide the operator in resecting thebone, and these planes are aligned with the mechanical axis.

According to step 6, an orientation of the positioning block 10 ismanually adjusted, as a function of the alterations to be performed onthe tibia.

According to step 7, alteration parameters such as varus/valgus, andflexion/extension are provided as calculated by the CAS as a function ofthe manual adjustments to orientation of the positioning block 10. TheCAS receives the tracking of the mechanical axis from the tracker member10′, as well as the orientation changes from the MEMS tracking circuitryon the positioning block 10. Therefore, the CAS deducts motion of thetibia from the orientation changes of the tracking circuitry tocalculate the implant parameters. The amount of varus/valgus andflexion/extension is updated in real-time on the positioning block anddisplayed to the surgeon by a simple graphical means. For example, anarray of light-emitting diodes (LEDs) can be positioned on thepositioning block or in the field of view of the surgeon such that agreen light goes on when the angle is appropriate and stays red as longas the orientation is not appropriate in a particular plane.

Alternatively, the tracker member 10′ could be eliminated from theprocedure, relying exclusively on the positioning block 10 to obtainmechanical axis information.

If no tracker member 10′ was used on the tibia during the cuttingprocedure, it could still be installed after the cut has been made inorder to provide HKA information later on during the procedure. Once thecut has been made, a tracker member 10′ would then be fixed to the tibiaand all coordinate system information registered to this tracker member10′ for further measurements, such as HKA.

Once the planes have been cut in the tibia, the positioning block may beused to digitize the orientation of the cut planes with respect to themechanical axis of the tibia. More specifically, as the positioningblock 75 is tracked for orientation by the MEMS unit 79, the positioningblock 75 may simply be laid upon the cut planes so as to digitize anorientation of such planes with respect to the mechanical axis of thetibia.

Once a desired orientation is set, the positioning block 10 is used toguide the operator in resecting the tibia as set forth in United StatesPublication No. 2008/0065084, and United States Publication No.2004/0039396.

As additional information, the MEMS trackable members 10′ on the femurand the tibia may be used concurrently to determine the HKA by lying theleg flat on a table. Alternatively, the femur and tibia may be held incomplete extension, with the leg held at an angle in space. Such amaneuver is simply accomplished by lifting the whole leg while holdingit from the talus. The micro-circuitry of tracking members installed onthe tibia and femur may be providing rotational information using atleast one three DOF sensor, such as a gyroscopic sensor. In such a case,the gyroscopic sensor can provide alignment information of the femurrelative to the tibia.

Referring to FIG. 6, a MEMS positioning block 10 and a MEMS trackablemember 10′ in accordance with an embodiment of the present applicationare generally shown as being fixed to a bodily element such as a bone B.

The MEMS positioning block 10 and the MEMS trackable member 10′ are usedwith a tracking CAS system and comprises tracking circuitry, andoptionally a wireless transmitter (or like communication circuitry). Theblock 10 and member 10′ may be wired to the CAS system as well.

In an embodiment of the present disclosure, the tracking circuitry isknown as a two-degree-of-freedom (hereinafter DOF) micro-circuitry, butmay alternatively provide data for more than three DOFs. The trackingcircuitry of the MEMS positioning block 10 and the MEMS trackable member10′ outputs orientation-based data pertaining to the bone B.

As an alternative embodiment, transmitters are connected to the trackingcircuitry of the MEMS positioning block 10 and the MEMS trackable member10′ so as to transmit the tracking data of the tracking circuitry 10 tothe processing system of the CAS system 100. The technology used for thetransmitter 10′ is selected to operate in a surgical environment, suchas RF. As an example, Bluetooth™, Zigbee™ or Wi-Fi transmitters areconsidered for their wide availability. The MEMS can be manufactured asa single disposable unit, possibly integrated to the positioning block10 and to the trackable member 10′. As an alternative embodiment,sensors can be configured to communicate necessary information betweenthemselves.

Referring to FIG. 6, a tracking computer-assisted surgery systemincorporating the MEMS positioning block 10 and the MEMS trackablemember 10′ is generally illustrated at 100. The computer-assistedsurgery system (CAS system) has a processing system 101, which typicallycomprises a computer having a processor. A receiver 102 is provided inthe processing system 101 so as to receive the orientation-based datasignal from the MEMS positioning block 10 and the MEMS trackable member10′. Alternatively, the MEMS positioning block 10 and the MEMS trackablemember 21 are wired to the processing system 101.

A controller 103 is connected to the receiver 102 or is wired to theMEMS positioning block 10 and the MEMS trackable member 10′. Therefore,the controller 103 receives the signal data from the receiver 102 orfrom the MEMS positioning block 10 and the MEMS trackable member 10′.

A signal interpreter 104 is used to convert the signal data receivedinto orientation data for the MEMS positioning block 10 and the MEMStrackable member 10′.

A geometry database 105 is provided in order to store the calibrationdata, and other intraoperative data such as the mechanical axis definedintraoperatively. The calibration data is therefore relational databetween the bone B, the MEMS positioning block 10 and the MEMS trackablemember 10′.

A parameter calculator 106 is associated with the controller 103. Theparameter calculator 106 receives the orientation data from the signalinterpreter 104, and the relational data from the geometry database 105.With the relational data provided by the database 105, the parametercalculator 106 calculates alteration parameters as a function of theorientation of the positioning block 10 with respect to the bone B, suchas varus/valgus and flexion/extension and the like, depending on theapplication. Accordingly, the controller 103 outputs alterationparameters to the user interface 110.

In an embodiment, either one of the MEMS positioning block 10 and theMEMS trackable member 10′ has a self-enclosed processing unit connectedto the tracking circuitry. The MEMS positioning block 10 or the MEMStrackable member 10′ has the tracking circuitry, a transmitter/receiverand also the processing system 101, all in a compact self-enclosedcasing. Accordingly, the transmitter/receiver 10′ is used to shareinformation with other one of the MEMS positioning block 10 and the MEMStrackable member 10′ used concurrently during the surgical procedure.

In such an embodiment, the alteration parameters are displayed directlyon the positioning block 10 or on the trackable member 10′. It isconsidered to use a set of LEDs or another form of compact electronicdisplay (e.g., LCD) as user interface 1, to minimize the size of theself-enclosed casing.

Referring to FIG. 7, a caliper in accordance with another embodiment isgenerally shown having a base L1, and arms L2 and L3. The caliper isused to determine length of objects using tracking circuitry such asMEMS. More specifically, the length of the base L1 is known, as is thelengths of the arms L2 and L3.

The arms L2 and L3 are pivotally mounted to ends of the base L1. Thefree ends of the arms L2 and L3 are used to identify a limit point ofthe object to measure. In other words, the distance measured is thedistance between the free ends of the arms L2 and L3.

The tracking circuits 10′ are secured to the arms L2 and L3, and produceorientation data pertaining to an orientation of the arms L2 and L3 in aplane in which the arms and the base L1 lie. The orientation data isillustrated as θ1 and θ2. Accordingly, the distance is calculated using:L1 +L2 sin(θ1)+L3 sin(θ2). The caliper may have a single arm L2 and baseL1 (i.e., no L3, simply removed from FIG. 7). In such a case, the anglebetween the arm L2 the and base L1 along with their lengths are used inbasic trigonometry to calculate the distance between the tips of L1 andL2.

The tracking circuitry is connected to the CAS system, or wirelesslytransmits data to a CAS system. Moreover, it is considered to provide atracking circuit on the base L1 as well, so as to obtain the orientationchanges of the arms L2 and L3 relative to the base L1.

The MEMS positioning block 10, the MEMS trackable member 10′ (FIG. 6)and the caliper (FIG. 7) may be disposable, reusable aftersterilization, or returnable for refurbishment and resterilization bythe manufacturer.

Referring to FIG. 11, an axis-digitizing device is generally shown at85. The axis-digitizing device 85 may be used as an alternative to theaxis-digitizing device 70 of FIG. 8. The device 85 has a base 86 thatanchors to the tibia at the knee so as to be aligned with theanterior-posterior axis of the tibia, and features an alignment bar 87projecting downwardly. The alignment bar 87 is to be aligned with theanterior crest of the medial third of the tibial tubercle.Alternatively, the bar 87 may be directed towards the 2^(nd) metatarsalbone. The device 85 may also be equipped with a self-centering mechanismat both ends, connecting to the center of the tibial plateau and to thecenter of the ankle joint. The MEMS unit 88 is integral with thealignment bar 87, whereby any change in orientation of the alignment bar87 is trackable. Knobs 89A and 89B are used to adjust the orientation ofthe alignment bar 87 with respect to the tibia.

Referring to FIGS. 12 and 13, a bracket 90 is shown as securing thetracking member 10′ and the positioning block 75 to the tibia, in anon-invasive manner. A translational joint 91 is provided in the bracket90 to ensure the vertical alignment of the positioning block 75 withrespect to the knee. In FIG. 12, the bracket 90 has two rotationaljoints, to provide orientation adjustments of the positioning block 75.It is considered to use joint encoders to measure any rotation of thepositioning block 75 with respect to the tracking member 10′. The jointencoders may be an alternative to the MEMS of the positioning block 75,or data to validate the information from the MEMS of the positioningblock 75.

As yet another alternative, it is considered to allow the operator toadjust a position/orientation of the positioning block 10/75 in afreehand mode. In such a case, the alteration parameters are displayedwhile the positioning block 10/75 is displaced with respect to the bone,so as to allow the operator to select a position/orientation along thesealteration parameters. Once an appropriate position/orientation thepositioning block 10/75 is pinned to the bone.

The invention claimed is:
 1. A caliper for determining a dimension of anobject, comprising: a base having a known base length; at least one armpivotally mounted to an end of the base, the arm having a known armlength, and having a free end used to identify a limit point of theobject to measure; an inertial sensor unit secured to the at least onearm, the inertial sensor unit producing orientation data pertaining toat least one degree of freedom in orientation of the arm in a plane inwhich the at least one arm and the base lie; whereby the dimensionbetween limit points is calculated from the known base length and thearm length and from the orientation data of the at least one armrelative to the base.
 2. The caliper according to claim 1, comprisingtwo of said arms with the free ends of the arms identifying the limitpoints of the object to measure, each said arm having an own one of theinertial sensor unit secured thereto.
 3. The caliper according to claim1, comprising one of said inertial sensor unit on one of the arms, andanother one said inertial sensor unit on at least one of the other armand the base.